The Sun is the most powerful UV source, and the living species of the Earth""s ecosystem are affected by the solar UV radiation. The ozone layer and other atmospheric gases strongly absorb the UV emission from the Sun, and only light with wavelengths longer than 280 nm reaches the Earth""s surface. The determination of the effects of the solar UV light on the terrestrial ecosystem and on human beings is an important subject, and has been driving the need for reliable and efficient visible-blind UV detectors.
The high-energy cut-off of UV (Al)GaN-based photovoltaic detectors is generally limited by the large absorption coefficient at high energies and the small minority carrier diffusion length. As a result, high-energy photons are absorbed in the cladding layer rather than in the space-charge region. Several design changes have been recently reported to overcome these limitations including the use of p-i-n instead of p-n junctions, employment of AlxGa1xe2x88x92xN instead of GaN windows, use of semitransparent recessed windows, and back-illuminated detector configurations. See G. Y. Xu, A. Salvador, W. Kim, Z. Fan, C. Lu, H. Tang, H. Morkoxc3xa7, G. Smith, M. Estes, B. Goldenberg, W. Yang, S. Krishnankutty, Appl. Phys. Lett. 71, 2154 (1997); T. Li, A. L. Beck, C. Collins, R. D. Dupuis, J. C. Campbell, J. C. Carrano, M. J. Schurman, I. A. Ferguson, Appl. Phys. Lett. 75, 2421 (1999); D. J. H. Lambert, M. M. Wong, U. Chowdhury, C. Collins, T. Li, H. K. Kwon, B. S. Shelton, T. G. Zhu, J. C. Campbell, R. D. Dupuis, Appl. Phys. Lett. 77, 1900 (2000); and J. M. Van Hove, R. Hickman, J. J. Klaassen, P. P. Chow, P. P. Ruden, Appl. Phys. Lett. 70, 2282 (1997).
Despite the considerable broadening of spectral range and the increase in peak responsivity attained due to the new designs (as described in the above listed references), a direct control of fundamental GaN transport properties, such as minority carrier diffusion length, has never been achieved. Minority carrier diffusion length is a distance covered by minority carriers due to diffusion, without recombination.
Current GaN-based device technologies include light-emitting diodes (LEDs), laser diodes, and UV detectors on the photonic side. The potential of the wide band gap, nitride-based, GaN and AlGaN semiconductors for use in optoelectronics has been well documented. See S. J. Pearton, J. C. Zolper, R. J. Shul, F. Ren, J. Appl. Phys. 86, 1 (1999). However, there are remaining problems.
As was already mentioned, the main difficulty that has been encountered for Schottky or p-n junction photovoltaic detectors is the reduced response at high energy, due to the large absorption coefficient and the small diffusion length, L, in GaN. Carriers are, therefore, generated close to the surface and recombine. In order to solve this problem, very thin p layers (for light incident on the p side of a p-n junction) must be used.
For optoelectronic device to be produced in the nitride semiconductors, improvements in p-type doping are needed. At present, Magnesium (Mg) is the only technologically feasible acceptor in (Al)GaN technology. See S. J. Pearton, J. C. Zolper, R. J. Shul, F. Ren, J. Appl. Phys. 86, 1 (1999).
In p-type (Al)GaN, similar to (Al)GaAs, several types of deep levels, located 1.1, 1.4, 2.04 eV above the valence band edge, were identified. See C. H. Qiu, J. I. Pankove, Appl. Phys. Lett. 70, 1983 (1997). These levels are likely related to Mg doping and are assumed to be responsible for the persistent photoconductivity behavior in III-Nitrides. See C. H. Qiu, J. I. Pankove, Appl. Phys. Lett. 70, 1983 (1997).
Various U.S. Patents have been proposed over the years. See for example, U.S. Pat. No. 3,864,174 to Akiyama et al.; U.S. Pat. No. 3,894,890 to Bauerlein et al.; U.S. Pat. No. 3,938,178 to Miura et al.; U.S. Pat. No. 4,065,780 to Ballantyne; U.S. Pat. No. 4,161,814 to Ballantyne; U.S. Pat. No. 4,210,464 to Tanaka et al.; U.S. Pat. No. 4,238,694 to Kimerling et al.; U.S. Pat. No. 4,275,404 to Cassidy et al.; U.S. Pat. No. 4,349,906 to Scifres et al.; U.S. Pat. No. 4,399,448 to Copeland; U.S. Pat. No. 4,414,558 to Nishizawa et al.; U.S. Pat. No. 4,454,526 to Nishizawa et al.; U.S. Pat. No. 4,585,489 to Hiraki et al.; U.S. Pat. No. 4,679,063 to White; U.S. Pat. No. 4,833,507 to Shimizu et al.; U.S. Pat. No. 5,510,274 to Minato; U.S. Pat. No. 5,808,352 to Sakamoto; and U.S. Pat. No. 5,858,559 to Barbour et al. However, none of the above listed patents overcome the problems with the prior art described above.
The primary objective of the invention is to provide a method and system for enhancing quantum efficiency, response, and spectral range in photon detectors and solar cells.
The secondary objective of the invention is to provide a method and system for controlling minority carrier diffusion length (transport) to improve the performance of optoelectronic devices.
A preferred method and system for enhancing quantum efficiency, response, and spectral range in photon detectors and solar cells is injection of electrons into a p-region of a photon device over a selected time period and controlling minority carrier diffusion length in the photon device to broaden spectral range and increase responsivity of the photon device. The invention can include periodically injecting electrons after several days, where the selected time includes approximately 10 seconds to approximately 1500 seconds. The electrons can be injected under a forward bias of the p-n junction, and can increase quantum efficiency between approximately 2 to approximately 5 fold.
The photon device can include a p-n junction detector. Additionally, the photon device can include a p-n junction solar cell. And still furthermore, the photon device can include a Schottky barrier detector.
The method and system can also automatically sense the performance output of the photon device with a computer type logic circuit, and provide a feedback signal to control the injection of the electrons.